The present invention relates to radio telecommunication systems. More particularly, and not by way of limitation, the present invention is directed to an apparatus and method for improving speech quality in a radio telecommunication system.
The Global System for Mobile Communications (GSM) network is expanding rapidly due to the increased demand for mobile voice service in emerging markets with densely populated cities and limited radio spectrum. To achieve this, the existing spectrum for circuit-switched voice channels must carry additional voice traffic. It is of great interest to operators to carry this increased voice traffic while maintaining acceptable voice quality.
To increase voice capacity, a new GSM feature called Voice services over Adaptive Multi-user channel on One Slot (VAMOS) is currently being standardized in the GSM EDGE Radio Access Network (GERAN) to multiplex two mobile terminals in speech mode onto the same traffic channel (TCH) using the same time slot and carrier frequency. Traditionally, the speech signals of different users are transmitted in different time slots. The speech payload bits are channel encoded, mapped to bursts, modulated using GMSK, rotated by π/2, filtered with a transmission (TX) pulse, and transmitted. VAMOS, however, combines two burst-mapped encoded speech data streams by using an adaptive quaternary constellation. To generate one such symbol, one bit from the first stream is mapped to the Most Significant Bit (MSB) and one bit from the second signal is mapped to the Least Significant Bit (LSB). These so-called “dibits” are mapped to a symbol of the constellation. This procedure is repeated until all bits of the two streams have been used. The resulting symbols are progressively rotated by π/2, filtered with the TX pulse, and then transmitted.
Each of the two co-TCH mobile stations extracts its own signal using its training sequence. The training sequences of the two mobile stations must exhibit low cross-correlation and good auto-correlation, and are preferably orthogonal. Since the signal of one user interferes with the signal of the other user, interference cancellation capability of the mobile receivers is highly desirable. It is advantageous that at least one of the two users has the Single Antenna Interference Cancellation (SAIC) capability. With SAIC, a mobile station requires a lower carrier-to-interference (C/I) ratio for correctly decoding received signals. This enables the base station to allocate additional power to the other mobile station which may need more power in order to stay on channel.
The sub-channel transmission power allocated to the two co-TCH users must be carefully controlled, especially on the downlink. However, the mobiles cannot send up-to-date downlink quality information to the base station because there is no fast control channel in the GSM system. It has been proposed to use RXQUAL to control the Sub-Channel Power Imbalance Ratio (SCPIR). RXQUAL is a signal quality level that is determined from the average Bit Error Rate (BER) calculated every 480 ms. The mobile stations send RXQUAL to the base station every 480 ms as part of their measurement reports.